Radar and Lidar Fundamentals and Applications. Radar image: Himalayas.
Radar Fundamentals - NPS
Transcript of Radar Fundamentals - NPS
Prof. David JennDepartment of Electrical & Computer Engineering
833 Dyer Road, Room 437Monterey, CA 93943
(831) [email protected], [email protected]
http://www.nps.navy.mil/faculty/jenn
Radar Fundamentals
2
Overview
• Introduction• Radar functions• Antennas basics• Radar range equation• System parameters• Electromagnetic waves• Scattering mechanisms• Radar cross section and stealth• Sample radar systems
3
• Bistatic: the transmit and receive antennas are at different locations as viewed from the target (e.g., ground transmitter and airborne receiver).
• Monostatic: the transmitter and receiver are colocated as viewed from the target (i.e., the same antenna is used to transmit and receive).
• Quasi-monostatic: the transmit and receive antennas are slightly separated but still appear to be at the same location as viewed from the target (e.g., separate transmit and receive antennas on the same aircraft).
Radio Detection and Ranging
TARGET
TRANSMITTER (TX)
RECEIVER (RX)
INCIDENT WAVE FRONTS
SCATTERED WAVE FRONTS
Rt
Rrθ
4
Radar Functions
• Normal radar functions:1. range (from pulse delay)2. velocity (from Doppler frequency shift)3. angular direction (from antenna pointing)
• Signature analysis and inverse scattering:4. target size (from magnitude of return)5. target shape and components (return as a function of direction)6. moving parts (modulation of the return)7. material composition
• The complexity (cost & size) of the radar increases with the extent of the functions that the radar performs.
5
Electromagnetic SpectrumWavelength (λ, in a vacuum and approximately in air)
10-3 10-2 10-1 1 10-5 10-4 10-3 10-2 10-1 1 101 102 103 104 105
109 108 107 106 105 104 103 102 10 1 100 10 1 100 10 1
Frequency (f, cps, Hz)
Radio
Microns Meters
Giga Mega Kilo
Microwave
Millimeter
InfraredUltraviolet
Visible
Optical
300 MHz300 GHz
EHF SHF UHF VHF HF MF LF
Typical radar frequencies
6
Radar Bands and Usage
8
(Similar to Table 1.1 and Section 1.5 in Skolnik)
7
Time Delay Ranging
• Target range is the fundamental quantity measured by most radars. It is obtained by recording the round trip travel time of a pulse, TR , and computing range from:
where c = 3x108 m/s is the velocity of light in free space.
TIMETR
AM
PLIT
UD
E
TRANSMITTED PULSE RECEIVED
PULSE
Bistatic: t r RR R cT+ =
Monostatic: ( )2R
t rcTR R R R= = =
8
Classification by FunctionRadars
Civilian MilitaryWeather Avoidance
Navagation & Tracking
Search & Surveillance
Space Flight
Sounding
High Resolution Imaging & Mapping
Proximity Fuzes
Countermeasures
9
Classification by WaveformRadars
CW Pulsed
Noncoherent Coherent
Low PRF Medium PRF
High PRF
FMCW
("Pulse doppler")CW = continuous wave FMCW = frequency modulated continuous wave PRF = pulse repetition frequency
Note: MTI Pulse Doppler
MTI = moving target indicator
10
Plane Waves
z
1t 2t
xE
DIRECTION OF PROPAGATION
oE
oE−
λ
• Wave propagates in the zdirection
• Wavelength, λ• Radian frequency ω = 2π f
(rad/sec) • Frequency, f (Hz)• Phase velocity in free space
is c (m/s)• x-polarized (direction of the
electric field vector)• Eo, maximum amplitude of
the wave Electric field vector
11
Wavefronts and Rays• In the antenna far-field the waves are spherical
• Wavefronts at large distances are locally plane
• Wave propagation can be accurately modeled with a locally plane wave approximation
PLANE WAVE FRONTS
RAYS
Local region in the far field of the source can be approximated by a plane wave
ANTENNA
RADIATIONPATTERN
D
R
2( 2 / )R D λ>
12
• If multiple signal sources of the same frequency are present, or multiple paths exist between a radar and target, then the total signal at a location is the sum (superposition principle).
• The result is interference: constructive interference occurs if the waves add; destructive interference occurs if the waves cancel.
• Example: ground bounce multi-path can be misinterpreted as multiple targets.
Superposition of Waves
thrh
rdtd
Grazing Angle,ψ
Airborne Radar Target
13
• Polarization refers to the shape of the curve traced by the tip of the electric field vector as a function of time at a point in space.
• Microwave systems are generally designed for linear or circularpolarization.
• Two orthogonal linearly polarized antennas can be used to generate circular polarization.
Wave Polarization
1
2
3
4
5
6
LINEAR POLARIZATIONELECTRIC FIELD
VECTOR AT AN INSTANT IN TIME
ORTHOGANALTRANSMITTING
ANTENNAS
ELECTRICFIELDS
HORIZONTAL, H
VERTICAL, V
HORIZONTAL ANTENNA RECEIVES ONLY HORIZONTALLY POLARIZED RADIATION
12
3
4
CIRCULAR POLARIZATION
14
Antenna Parameters
• Gain is the radiation intensity relative to a lossless isotropicreference.
• Fundamental equation for gain:
• In general, an increase in gain is accompanied by a decrease in beamwidth, and is achieved by increasing the antenna size relative to the wavelength.
• With regard to radar, high gain and narrow beams are desirable for long detection and tracking ranges and accurate direction measurement.
24 /, effective area
= aperture area efficiency (0 1)
/ , wavelength
ee
G AA AA
c f
π λε
ε ελ
==
= ≤ ≤=
Low gain High gain(Small in wavelengths) (Large in wavelengths)
ANTENNA DIRECTIONAL RADIATION PATTERN
Aperture area
15
• Half power beamwidth, HPBW (θB) • Polarization• Sidelobe level • Antenna noise temperature (TA) • Operating bandwidth • Radar cross section and other signatures
Antenna Parameters
0
MAXIMUM SIDELOBE
LEVEL
PEAK GAIN
GA
IN (d
B)
θsPATTERN ANGLE
θ
SCAN ANGLE
HPBW3 dB
0
MAXIMUM SIDELOBE
LEVEL
PEAK GAIN
GA
IN (d
B)
θsPATTERN ANGLE
θ
SCAN ANGLE
HPBW3 dB
Rectangular dB pattern plot
G
0.5G
G
0.5G
Polar voltage pattern plot
16
• Airborne applications: > Size, weight, power consumption> Power handling> Location on platform and required field of view > Many systems operating over a wide frequency spectrum> Isolation and interference> Reliability and maintainability> Radomes (antenna enclosures or covers)
• Accommodate as many systems as possible to avoid operational restrictions (multi-mission, multi-band, etc.)
• Signatures must be controlled: radar cross section (RCS), infrared (IR), acoustic, and visible (camouflage)
• New antenna architectures and technologies> Conformal, integrated> Digital “smart” antennas with multiple beams> Broadband
Radar Antenna Tradeoffs
17
Radar Range Equation
• Quasi-monostatic
2
transmit power (W)received power (W)transmit antenna gainreceive antenna gain
radar cross section (RCS, m ) effective aperture area of receive antenna
trtr
er
PPGG
Aσ
====
==
R
TX
Pt
Gt
RX
Pr
Gr σ
Pr =PtGtσAer(4πR2 )2 =
PtGtGrσλ2
(4π )3 R4
18
Minimum Detection Range
• The minimum received power that the radar receiver can "sense" is referred to a the minimum detectable signal (MDS) and is denoted .
• Given the MDS, the maximum detection range can be obtained:
SminR
PrPr ∝1/ R4
Rmax
Smin
Pr = Smin =PtGtGrσλ2
(4π )3 R4 ⇒ Rmax =PtGtGrσλ2
(4π )3 Smin
⎛
⎝ ⎜
⎞
⎠ ⎟
1 / 4
19
Radar Block Diagram
• This receiver is a superheterodyne receiver because of the intermediate frequency (IF) amplifier. (Similar to Figure 1.4 in Skolnik.)
• Coherent radar uses the same local oscillator reference for transmit andreceive.
20
Coordinate Systems
• Radar coordinate systemsspherical polar: (r,θ,φ)azimuth/elevation: (Az,El) or
• The radar is located at the origin of the coordinate system; the Earth's surface lies in the x-y plane.
• Azimuth (α) is generally measured clockwise from a reference (like a compass) but the spherical system azimuth angle (φ ) is measured counterclockwise from the x axis. Therefore
(α ,γ )
α = 360 −φγ = 90 −θ
CONSTANT ELEVATION
x
y
z
θ
φγ
ZENITH
HORIZON
P
α
r
Target
Constant El cut
Constant Az cut
Radar
21
Radar Display Types
RANGE (TIME)
REC
EIV
ED P
OW
ER
TARGET RETURN
AZIMUTH
RA
NG
E
0-180 180
TARGET BLIP
"A" DISPLAY "B" DISPLAY
"C" DISPLAYPLAN POSITION INDICATOR (PPI)
AZIMUTH
ELEV
ATI
ON
0-180 180
TARGET BLIP
0
90RANGE UNITS
RADAR AT CENTER
AZIMUTH
TARGET BLIP
22
Pulsed Waveform• In practice multiple pulses are transmitted to:
1. cover search patterns2. track moving targets3. integrate (sum) several target returns to improve detection
• The pulse train is a common waveform
TIME
τ
Po
T p
peak instantaneous power (W)pulse width (sec)
1/ , pulse repetition frequency (PRF, Hz)interpulse period (sec)
number of pulses
o
p p
p
P
f TTN
τ=
====
23
Range Ambiguities
• For convenience we omit the sinusoidal carrier when drawing the pulse train
• When multiple pulses are transmitted there is the possibility of a range ambiguity.
• To determine the range unambiguously requires that . The unambiguous range is
TIME
τ
Po
T p
TIME
TRANSMITTED PULSE 1
TRANSMITTED PULSE 2
TARGET RETURN
TR1
TR2
Tp ≥2Rc
Ru =cTp
2=
c2 fp
24
Range Resolution
• Two targets are resolved if their returns do not overlap. The range resolution corresponding to a pulse width τ is .∆R = R2 − R1 = cτ /2
cτ / 2
cτ
cτ / 2TIME STEP 1 TIME STEP 2
TIME STEP 3 TIME STEP 4
to to +τ /2
to + τ to +3τ /2
R1
R2
R1
R1 R1
R2
R2R2
TARGET
25
Range Gates
• Typical pulse train and range gates
• Analog implementation of range gates
L1 2 3 M
L
1 2 3 M
L
1 2 3 M
L
1 2 3 M
L
DWELL TIME = N / PRF
M RANGE GATES
t
TRANSMIT PULSES
RECEIVER
.
M
M
.....
...
..
M
M.....
...
..
.
TO SIGNAL PROCESSOR
OUTPUTS ARE CALLED "RANGE BINS" • Gates are opened and closed sequentially
• The time each gate is closed corresponds to a range increment • Gates must cover the entire interpulse period or the ranges of interest • For tracking a target a single gate can remain closed until the target leaves the bin
26
Clutter and Interference
TX
RX
TARGET
GROUND
MULTIPATHDIRECT PATH
CLUTTER
INTERFERENCE
ANTENNA MAIN LOBE
GROUNDSIDELOBE CLUTTER IN RANGE GATE
RANGE GATE
SPHERICAL WAVEFRONT (IN ANTENNA FAR FIELD)
TARGET
RAIN (MAINBEAMCLUTTER)
GROUND (SIDELOBECLUTTER)
The point target approximation is good when the target extent << ∆R
27
Thermal Noise
• In practice the received signal is "corrupted" (distorted from the ideal shape and amplitude) by thermal noise, interference and clutter.
• Typical return trace appears as follows:
• Threshold detection is commonly used. If the return is greater than the detection threshold a target is declared. A is a false alarm: the noise is greater than the threshold level but there is no target. B is a miss: a target is present but the return is not detected.
TARGET RETURNS
TIME
REC
EIV
ED P
OW
ER
RANDOM NOISE
DETECTION THRESHOLD
(RELATED TO S )min
AB
28
Thermal Noise Power
• Consider a receiver at the standard temperature, To degrees Kelvin (K). Over a range of frequencies of bandwidth Bn (Hz) the available noise power is
where (Joules/K) is Boltzman's constant.• Other radar components will also contribute noise (antenna, mixer,
cables, etc.). We define a system noise temperature Ts, in which case the available noise power is
No = kToBn
231.38 10Bk −= ×
No = kTsBn
TIME OR FREQUENCY
NOISE POWER
29
Signal-to-Noise Ratio (SNR) • Considering the presence of noise, the important parameter for detection is
the signal-to-noise ratio (SNR)
• Factors have been added for processing gain Gp and loss L• Most radars are designed so that • At this point we will consider only two noise sources:
1. background noise collected by the antenna (TA)2. total effect of all other system components (To, system effective noise temperature)
2
3 4SNR(4 ) T
t t r pr
o B s n
PG G G LPN R k B
σλ
π= =
Ts = TA + Te
1/nB τ≈
30
Integration of Pulses• Noncoherent integration (postdetection
integration): performed after the envelope detector. The magnitudes of the returns from all pulses are added. SNR increases approximately as .
• Coherent integration (predetectionintegration): performed before the envelope detector (phase information must be available). Coherent pulses must be transmitted. The SNR increases as N.
• The last trace shows a noncoherentintegrated signal.
• Integration improvement an example of processing gain.
N
From Byron Edde, Radar: Principles, Technology, Applications, Prentice-Hall
31
Dwell Time
HALF POWER ANGLE
HPBW ...
MAXIMUM VALUE OF
GAIN
ANTENNA POWER PATTERN (POLAR PLOT)
Bθ
DB /λθ ≈
• Simple antenna model: constant gain inside the half power beamwidth(HPBW), zero outside. If the aperture has a diameter D with uniform illumination .
• The time that the target is in the beam (dwell time, look time, or time on target) is tot
• The beam scan rate is ωs in revolutions per minute or in degrees per second.
• The number of pulses that will hit the target in this time is
sBt θθ &=ot
ss
dtd
θθ &=
pB ftn ot=
32
Doppler Shift
• Targets in motion relative to the radar cause the return signal frequency to be shifted.
• A Doppler shift only occurs when the relative velocity vector has a radial component. In general there will be both radial and tangential components to the velocity
•• •1 2 3 vr
wave frontsexpanded
1 2 4 3 4 wave frontscompressed
1 2 4 3 4
WAVE FRONT EMITTED AT POSITION 1
WAVE FRONT EMITTED AT POSITION 2
R decreasing ⇒dRdt
< 0 ⇒ fd > 0 (closing target)
R increasing ⇒ dRdt
> 0 ⇒ fd < 0 (receeding target)
r v t
r v r
r v
•R
2 /d rf v λ= −
33
Doppler Filter Banks
• The radar’s operating band is divided into narrow sub-bands. Ideally there should be no overlap in sub-band frequency characteristics.
• The noise bandwidth of the Doppler filters is small compared to that of the radar’s total bandwidth, which improves the SNR.
• Velocity estimates can be made by monitoring the power out of each filter. • If a signal is present in a filter, the target's velocity range is known.
f c
ff c + f d
AMP FREQUENCY CHARACTERISTIC
NARROWBAND DOPPLER FILTERS CROSSOVER
LEVEL
dB S
CA
LE
34
Velocity Ambiguities
ωωωc ωc +ωd
Spectrum of doppler shifted CW signal
Coherent pulse train spectrum (fixed target -- no doppler)
ωc
ωc +ωd
ω
ωc
CENTRAL LOBE
FILTER f d observed =2vrλ
mod(PRF)
f d = n PRF + f d apparent
Expanded central lobe region with target doppler shift
DOPPLER SHIFTED TARGET
RETURNS
• The spectrum is the Fourier transform of the pulse train waveform.
1/PRF
1/fp
35
Low, High, Medium PRF
• If fd is increased the true target Doppler shifted return moves out of the passband and a lower sideband lobe enters. Thus the Doppler measurementis ambiguous.
• PRF determines Doppler and range ambiguities:PRF RANGE DOPPLERHigh Ambiguous UnambiguousMedium Ambiguous AmbiguousLow Unambiguous Ambiguous
ω
ωc +ωdωc
APPARENT DOPPLER
SHIFTACTUAL DOPPLER
SHIFTf d max = ± f p / 2
vu = λ f d max / 2= ±λ f p / 4
∆vu = λ f p / 2
36
Track Versus Search• Search radars
> Long, medium, short ranges (20 km to 2000 km)> High power density on the target: high peak power, long pulses, long
pulse trains, high antenna gain> Low PRFs, large range bins > Search options: rapid search rate with narrow beams or slower search
rate with wide beams• Tracking radar
> Accurate angle and range measurement required> Minimize time on target for rapid processing> Special tracking techniques: monopulse, conical scan, beam switching
SUM BEAM, Σ
DIFFERENCE BEAM, ∆
SIGNAL ANGLE OF ARRIVAL
POINTING ERROR
SUM BEAM, Σ
DIFFERENCE BEAM, ∆
SIGNAL ANGLE OF ARRIVAL
POINTING ERROR
MonopulseTechnique
37
Antenna Patterns
• Fan beam for 2-d search
• Pencil beam for trackingfor 3-d search
38
Attack Approach
• A network of radars are arranged to provide continuous coverage of a ground target.
• Conventional aircraft cannot penetrate the radar network without being detected.
GROUND TARGET
ATTACK APPROACH
FORWARD EDGE OF BATTLE AREA (FEBA)
Rmax
RADAR DETECTION RANGE, Rmax
39
Radar Jamming
• The barrage jammer floods the radar with noise and therefore decreases the SNR.
• The radar knows it is being jammed.
GROUND TARGET
AIR DEFENSE RADAR
ATTACK APPROACH
STANDOFF JAMMER
RACETRACK FLIGHT PATTERN
40
Low Observability
GROUND TARGET
AIR DEFENSE
RADAR
ATTACK APPROACH
• Detection range depends on RCS, , and therefore RCS reduction can be used to open holes in a radar network.
• There are cost and performance limitations to RCS reduction.
Rmax ∝ σ4
41
Radar Cross Section (RCS)
• Typical values:
• Fundamental equation for the RCS of a “electrically large”perfectly reflecting surface of area A when viewed directly by the radar
• Expressed in decibels relative to a square meter (dBsm):
-40 -20 0 20 40 dBsm
m20.0001 0.01 1 100 10000
INSECTS BIRDS CREEPING & TRAVELING WAVES
FIGHTER AIRCRAFT
BOMBER AIRCRAFT
SHIPS
2
24 Aπ
σλ
≈
σdBsm =10log10(σ )
42
RCS Target Types
• A few dominant scatterers (e.g., hull) and many smaller independent scatterers
• S-Band (2800 MHz), horizontal polarization, maximum RCS = 70 dBsm
43
RCS Target Types
• Many independent random scatterers, none of which dominate (e.g., large aircraft)
From Skolnik• S-Band (3000 MHz)• Horizontal Polarization • Maximum RCS = 40 dBsm
44
Scattering Mechanisms
Double diffraction from sharp corners
Diffraction from rounded object
SPECULAR
DUCTING, WAVEGUIDE MODES
MULTIPLE REFLECTIONS
EDGE DIFFRACTION
SURFACE WAVES
CREEPING WAVES
• Scattering mechanisms are used to describe wave behavior. Especially important at radar frequencies:specular = "mirror like" reflections that satisfy Snell's lawsurface waves = the body surface acts like a transmission line diffraction = scattered waves that originate at abrupt discontinuities
45
Example: Dipole and Box• f =1 GHz, −100 dBm (blue) to −35 dBm (red), 0 dBm Tx power, 1 m metal cube
ANTENNA
BOX
Reflected Field Only
REFLECTED Incident + Reflected
Reflected + Diffracted
Incident + Reflected + Diffracted
ANTENNA
BOX
Reflected Field Only
REFLECTED Incident + Reflected
Reflected + Diffracted
Incident + Reflected + Diffracted
46
RCS Reduction Methods
• Shaping (tilt surfaces, align edges, no corner reflectors)• Materials (apply radar absorbing layers)• Cancellation (introduce secondary scatterers to cancel the “bare”
target)
From Fuhs
47
AN/TPQ-37 Firefinder• Locates mortars, artillery, rocket launchers and missiles • Locates 10 weapons simultaneously • Locates targets on first round • Adjusts friendly fire • Interfaces with tactical fire • Predicts impact of hostile projectiles• Maximum range: 50 km • Effective range:
Artillery: 30 km, Rockets: 50 km• Azimuth sector: 90°• Frequency: S-band, 15 frequencies• Transmitted power: 120 kW• Permanent storage for 99 targets; field exercise mode; digital data
interface
48
SCR-270 Air Search Radar
49
SCR-270-D-RADAR
• Detected Japanese aircraft approaching Pearl Harbor• Performance characteristics:
SCR-270-D Radio Set Performance Characteristics (Source: SCR-270-D Radio Set Technical Manual, 1942)Maximum Detection Range . . . . . . . . . . . . . . . . . . . . . . 250 milesMaximum Detection altitude . . . . . . . . . . . . . . . . . . . . . 50,000 ftRange Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 miles*Azimuth Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 degreesOperating Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . 104-112 MHzAntenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Directive array **Peak Power Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 kwPulse Width . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15-40 microsecondPulse Repetition Rate . . . . . . . . . . . . . . . . . . . . . . . . . . 621 cpsAntenna Rotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . up to 1 rpm, maxTransmitter Tubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 tridoes***Receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . superheterodyneTransmit/Receive/Device . . . . . . . . . . . . . . . . . . . . . . . spark gap
* Range accuracy without calibration of range dial.** Consisting of dipoles, 8 high and 4 wide.*** Consisting of a push-pull, self excited oscillator, using a tuned cathode circuit.
50
AN/SPS-40 Surface Search
• UHF long range two-dimensional surface search radar
51
AN/SPS-40 Surface Search
• UHF long range two-dimensional surface search radar. Operates in short and long range modes
• Range Maximum: 200 nmMinimum: 2 nm
• Target RCS: 1 sq. m.• Transmitter Frequency:
402.5 to 447.5 MHz• Pulse width: 60 s• Peak power: 200 to 255 kW• Staggered PRF: 257 Hz (ave)• Non-staggered PRF: 300 Hz
• AntennaParabolic reflectorGain: 21 dBHorizontal SLL: 27 dBVertical SLL: 19 dB HPBW: 11 by 19 degrees
• Receiver10 channels spaced 5 MHzNoise figure: 4.2 IF frequency: 30 MHzPCR: 60:1Correlation gain: 18 dBMDS: −115 dBmMTI improvement factor: 54 dB